Human factor I is a serine proteinase of the complement system which together with other regulatory components modulates activation of C3 and catabolism of C3b and C4b. It is essential in the regulation of most biological activities mediated by complement. Congenital human factor I deficiency, as a result, manifests an increased susceptibility to pyogenic infections and abnormalities of many complement mediated functions. Biosynthesis of factor I and its cDNA have been characterized in earlier studies permitting in this proposal to study: 1. Detailed structure of human factor I gene; 2. Tissue specific regulation of factor I gene expression; 3. Coordinate regulation of factor I and physiologically related genes (C3, C4, C4BP, and factors B and H); 4. Molecular basis of factor I deficiency in humans. These studies will include precise chromosomal localization of factor I gene on chromosome 4; identification of nuclear precursors of factor I mRNA; and characterization of polymorphic variants of the factor I protein and gene. Most of the techniques utilized in this proposal are routinely used in our lab, and include genomic library screening; restriction and Sl nuclease mapping; DNA sequencing; isolation of DNA and mRNA; Northern and Southern blot hybridization; metabolic labelling of cell cultures; immnoprecipitation and SDS-PAGE analysis.